Signal conditioning apparatus

ABSTRACT

A signal conditioning system that receives inputs from at least one pair of conductors connected to its input. Each such input is processed by an input filter and presented to a buffer amplifier. Each such input filter and buffer amplifier refers to and is powered by independent power sources whose power return reference potentials are independently determined by the potential of the corresponding input signal potential reference conductor for the signal frequencies of interest. The outputs of all such buffer amplifiers, the power return reference potentials, and the power return reference potential of the conditioning circuit output are all appropriately added or subtracted in the next circuit stage. This circuit stage consists of an amplifier buffer having low output impedance which is powered by another independent power source whose power return reference potential is independently determined by the potential of the output signal reference conductor. The output of this circuit stage is connected to an output inductor circuit which in turn drives the output signal conductor. The output includes a filter, and is designed to decouple unstable loading conditions while rejecting external influences on the output signal. The invention also includes means that connect the reference potential of the destination of the output conductors to the system power ground potential. The present invention provides a relatively inexpensive and efficient way of reducing or eliminating interference caused by coax cabling in audio, power and video amplifiers, for example.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 371 of PCT/US95/05293 filed Apr. 28, 1995 and acontinuation-in-part of application Ser. No. 08/234,343 filed Apr. 28,1994, now U.S. Pat. No. 5,436,593, which is a continuation-in-part ofapplication Ser. No. 07/879,941, filed May 8, 1992, now U.S. Pat. No.5,386,148.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to circuits that use signal conditioningcircuitry to eliminate interferences caused by magnetic fields, electricfields, and electro-magnetic or radio frequency fields on conductorsthat provide electrical connection between devices in a system. Thepresent invention also relates to various electronic circuit that driveconductors, the electronic circuits including signal conditioningcircuit to overcome the adverse effects of their loading on the signalsource. More specifically, the present invention relates to audioamplifiers, power amplifiers, video amplifiers, etc. that use signalconditioning circuitry for achieving the above-noted benefit.

2. Description of Related Art

Conductors that provide electrical connection between devices in asystem are often the source of many types of electrical interference.Magnetic fields, electric fields and electro-magnetic or radio frequencyfields are known to interfere with the fidelity of signals conveyed overconductors which are subjected to those fields. Furthermore, the groundor reference conductor of a typical signal carrying pair of conductorsare often connected to different local ground potentials between one endof the conductor as compared to the other, and currents are known toflow in such conductors which then produce voltage drops on thatconductor which also interfere with the fidelity of the signals beingconveyed. In addition, these conductors, especially when very long,present loads to the signal source that may adversely effect thefidelity of the signal.

The problems of conveying signals over conductor pairs in various typesof electronic circuits including amplifier systems such as audioamplifier systems, power amplifier systems, video amplifier systems,etc. is well known. The conveyance of signals, especially betweenpowered devices, is often plagued by electro-magnetic interference.

One method employed to reduce these interferences modulates the signalso that it can be easily separated from the interference, and thendemodulates the signal at the destination. For example, ananalog-to-digital converter can be utilized to convey digital impulsesover the connecting conductors instead of analog voltage potentials. Thedestination device in such instances must then convert the signal backto an analog signal potential. Such approaches, while effective, can bevery costly, and require extensive circuitry at both the sending andreceiving ends of the conductors. Such methods are exemplified by U.S.Pat. No. 4,922,536 to Hogue.

Another common method to reduce these interferences is to convey suchsignals in a differential manner. A common approach utilizes a threeconductor shielded cable where two of the conductors deliver the signaland its arithmetic inverse, and a third conductor, usually a shield,conveys the ground reference potential voltage. The conditioningcircuit, usually placed at the destination end of the conductors, formsthe difference between the potential of the first signal carryingconductor and the second signal carrying conductor. In theory, bothconductors are subject to the same interferences, and the subtraction ofthe signals as conveyed will eliminate the common mode noises. Thisapproach, while effective in eliminating most interference isnevertheless expensive and difficult to implement. To adapt thisapproach in the general case of processing signals between subsystemsrequires active circuitry at the sending end to form the inverse signal,and a separate active circuit at the receiving end to subtract thesignals. Multiple conductors are also required to be contained within asingle shield, which is more costly than conductors having only oneconductor surrounded by a shield. Such methods do not, however, addressany interference or other affects of the cables that connect thetransmitter and receiver to source and destination respectively. Suchmethods are exemplified by U.S. Pat. No. 4,979,218 to Strahm, and isdescribed at pages 69-71 of “GROUNDING AND SHIELDING TECHNIQUES ININSTRUMENTATION”, by Ralph Morrison, 3rd Edition, 1986,Wiley-Interscience.

One source of interference in the conveyance of these differentialsignals between electronic subsystems is referred to as the ground loop.Because it is common for there to be multiple electronic paths betweenthe reference potentials of each subsystem, and since such pathscommonly include sources of interference, these alternative paths areoften responsible for the interference present in those systems. Suchground loops are generally overcome by eliminating any electricalconnection by conductors between the subsystems. “GROUNDING ANDSHIELDING TECHNIQUES IN INSTRUMENTATION” by Morrison describes theelimination of the effects of the electrical connections betweensubsystems that convey their signals by differential means through theuse of tandem differential amplifiers powered by electrically isolatedpower supplies.

The first differential amplifier in the Morrison reference calculatesthe difference between the signals being conveyed, and the seconddifferential amplifier adds the reference potential of the destinationto the result of the first differential amplifier. The result is thatthe reference potentials of the source of the differential signal maydiffer from the reference potential of the destination without effectingthe expression of the signal at the destination. However, such anapproach is not easily adapted to electronic systems consisting ofsingle ended two wire signal conductors. Consequently, this approachsuffers from the same limitations as devices that convey signals bydifferential means. For example, there are no means suggested inMorrison for the elimination or suppression of the magnetic fieldinterference that may be picked up between the two conductors enclosedin the shield, due to differences in the magnetic field voltages inducedin those conductors. Moreover, Morrison does not address the pickup ofelectric field interference or any other cable affects due to the outputcable.

The circuits shown in the Morrison reference are also particularlysubject to the variation of op-amp characteristics. In particular theoutput impedance of the opamps used to determine A1 will negativelyimpact the interference rejection of any common mode voltage differencesbetween source and destination reference potentials as that impedancerelates to the difference resistors of gain stage A2. As this circuitcharacteristic is extremely gain and temperature dependent, suchinaccuracies are not easily controlled without increased expense in thedesign of the output stages of those circuits or without compromisesinherent in the utilization of higher impedances than would beappropriate in achieving other performance objectives such as thermalnoise and bandwidth which are adversely affected by higher resistorvalues in this case.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide various types ofelectronic circuits including amplifier systems, such as audio, powerand video amplifier systems, with circuitry to suppress or eliminate theexpression of all types of interference in the wiring conveying analogvoltage potential signals from a source to a destination. This isaccomplished in the present invention by the unique combination of novelinterference rejection circuits that address the sources of interferencein these systems in a less costly and more efficient manner than otherapproaches.

It is another objective of the present invention to remove any effectthat the loading of such wiring or the effects of the loading of thedestination in these devices may have upon the accuracy of the signal asconveyed by the source of the signal.

It is a further objective of the present invention to accomplish thepreceding objectives with a minimum number of precision resistorsproducing greater effective rejection than the prior art for a givencost.

An additional objective of the present invention is the reduction insensitivity of the circuit action to the characteristics of the gaincircuits and/or operational amplifier circuits employed by the circuitto achieve the various aims herein described.

Another objective of the present invention is to provide an economicalmeans of adjusting gain in these systems without affecting the resultinginterference rejection in practical applications.

Yet another objective of the present invention is to afford greaterrejection of electric field interference and any electric field affects,such as dielectric absorption, due to output cable physics.

A further objective of the present invention is to effect theseobjectives without altering the accuracy or fidelity of the signal(s)being conveyed between subsystems.

Another objective of the present invention is to provide a device whichaccomplishes every objective of the present invention as describedforthwith by means of an independent circuit which can easily beinserted into the existing wiring between popular electronic devicessuch as the above-described amplifier systems, for example, and whichcan accomplish every objective of the present invention as describedforthwith with a minimum amount of time required to install the devicein these systems.

It is a further objective of the present invention to provide all of thefunctions associated with state of the art amplification systems whileachieving the above-noted benefits with a minimum amount of additionalcircuitry and without adding any additional active circuitry in thesignal path.

A further objective of the present invention is to provide for theconditioning of single ended or differential signals with the samecircuit organization and interconnecting wire cable(s).

A further objective of the present invention is to provide for theconditioning of differential signals, or any number of signals, whereeach signal is produced with reference to independent potentialreferences which make higher levels of interference rejection possible.

It is also an objective of the present invention to accomplish everyobjective of the invention as described forthwith while utilizing signalwiring between devices which consists of two conductors arrangedconcentrically. This type of cabling is known as “COAX” which is ashortening of the term “CO-AXIAL”, and which refers to a cable whosecircular conductors share the same major axis.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the drawing:

FIG. 1 is FIGS. 1A and 1B together constitute a circuit diagram of apreferred embodiment of the signal conditioning circuitry to be includedin various types of electronic circuits according to various embodimentsof the present invention;

FIG. 2 is FIGS. 2A and 2B together constitute a circuit diagram of atypical audio amplifier;

FIG. 3 is FIGS. 3A and 3B together constitute a circuit diagram of thetypical audio amplifier including signal conditioning circuitryaccording to an embodiment of the present invention;

FIG. 4 is a block diagram of an amplifier system according to anembodiment of the present invention; and

FIG. 5 is a block diagram of another embodiment of the amplifier systemaccording to an embodiment of the present invention;

FIG. 6 is a block diagram of a video circuit including signalconditioning circuitry according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, in which like reference numerals identifyidentical or similar elements, and in particular to FIG. 1FIGS. 1A and1B, signal source 1 may be understood to be differentially related tosignal source 26 in instances where signal 1 and signal 26 are bothavailable and known to relate to each other in an arithmetically inversemanner. Such a relationship is not necessary to carry out the presentinvention. Other relationships may be appropriate; indeed, if thecircuit were to be used as a mixer of signals, there may be norelationship at all. Also, signal 1 or signal 26 need not convey asignal at all, and either may be disconnected to apply the device tosingle ended signals utilizing only two conductors to convey a signal,without limiting the effectiveness of the device.

Signal 1 and Signal 26 are conveyed over coaxial cable(s) 4 and 29respectively. Coaxial cables are preferred in connection with thisinvention because the coaxial nature of the cables offers the specialbenefit that both conductors respond nearly identically to magneticfield interference by virtue of the high degree of axial symmetrypossible around the major axis of the cable run. By virtue of thissymmetry, the outer conductor will respond to magnetic fields insubstantially the same way as the inner conductor will in accordancewith Lenz's law which relates an induced voltage to the rate of changeof magnetic field strength. Hence magnetic field disturbances in thevicinity of said cables will induce substantially the same voltage inthe outer conductor as such interference will induce in the innerconductor.

The present invention is particularly effective at rejecting theinfluence of any electric currents conducted by each shield that wouldtend to shift the reference potential of each signal at its source. Thisis often a problem because the ground potential impedance of the sourceis less than ideal in practice. Since each signal is generated withrespect to its own reference potential more exactly than with respect toany shared reference potential, and since all such current will onlysubstantially flow into that source reference impedance, affecting boththe signal source ground potential and signal potential substantiallyequally, the present invention will be very effective in eliminatinginterference so induced. The extreme high impedances of the power supplycircuit in the preferred embodiment provides for this principle in anovel and especially effective way at a much lower cost than othertechniques.

This power supply circuit is developed in the preferred embodiment byvery high impedance current source circuits composed of field effecttransistors 47, 22, 50 and 24 in combination with current programmingresistors 46, 21, 51 and 25 and opamps 48, 23, 52 and 26. The opampsprovide the biasing necessary for the transistors to conduct exactlythat current required to produce that voltage across the resistors thatmatch the biasing voltages produced by zener diodes 80 and 85 incombination with resistors 82 and 84. Capacitors 81 and 86 are includedto further reject any interference which may be present on the powersupply as provided by contacts 91, 89 and 90. In this way an exactcurrent is precisely metered through each transistor to operate opamps13, 16, 38, and 41. Because this current is metered so precisely, anychanges in potential at the opamp supply pins will have no bearing onthe current delivered. As a result, the effective impedance of the powersupply will be extremely high. The quality of this high impedancecurrent source is limited only by the gate-to-drain capacitance of thefield effect transistors used, which can be very small depending on thedevice characteristics. Field effect transistors with capacitances onthe order of 0.05 pf are known to exist in metal gate transistorsdesigned for VHF mixers. Such transistors would easily provide 3 megs ofequivalent isolation at 1 Mega-Hz provided opamps 13, 16, 38 and 41could handle that frequency accurately.

Other transistors could be used as well. While field effect transistorsare preferred, ordinary PNP and NPN transistors have proven to worksatisfactorily, but available device characteristics are not as ideal asfield effect transistors for this purpose. General purpose small signaltransistors are limited in isolation by the offset of the base currentwhich varies with collector voltage, and by larger base to collectorcapacitances, two effects which compromise the performance of thiscircuit. Careful selection of high B types or darlington configurationscan go a long way towards improving such circuits.

Each signal from source 1 or 26 is then subjected to resistor 5 and 30respectively. This resistance is provided primarily as a path for theinput bias current necessary for the operation of operational amplifiers13 and 38. Such resistors are not, however, required to carry out thepresent invention, yet are preferred to enable the device to carry outthe present invention in every possible context. For example, a commoncontext for the device involves an input signal source that is blockedwith a coupling capacitor which would not permit the necessary biascurrent to flow in the steady state without resistor 5 and 30. Theseresistors should be designed with the highest resistance value practicalconsidering the effect of the resistor on the source electronics andcable characteristics engendered in sources 1 and 26 and cable 4 and 29.

Capacitors 6, 9, 33 and 34 along with resistors 8 and 32 and inductors 7and 31 are all part of a Pi-filter arrangement designed to rejectfrequencies far higher than the frequencies of interest in the signal.Such a filter is preferred to prevent such high frequency interferencefrom interacting with various non-linear elements in typical operationalamplifiers, such as are indicated as 13 and 16 in the drawings, and soprevent the demodulating or converting of such high frequencyinterference to frequencies that would otherwise interfere with thefrequencies of interest.

Opamps 13 and 38 along with resistors 10, 11, 35 and 36 form bufferamplifiers which may be designed to include gain according to the ratioof the values of resistors 11 and 36 with respect to resistors 10 and35. Because the gain of these amplifiers may be important in instanceswhere differential signals are conditioned, these gains should be wellmatched. However, in the case of a single input, or multiple unrelatedinputs, the precision of the gain of this stage will not be importantbecause each input and each amplifier corresponding to each input refersto its own ground reference potential separately. In order to refer toeach ground reference potential separately, such as local groundpotentials as indicated as 3 and 28, each such buffer opamp 13 and 38must be powered by separate and independent sources of power whoseground return potentials can assume any value. Signal ground referencepotentials 3 and 28 may present by way of the ground conductors of cable4 and 29. In this way the opamp circuit can not inject any currents intothe signal carrying conductor, which would otherwise be possible throughvarious stray capacitances and internal opamp circuits if the opamp'spower supplies did not track the reference ground potential of eachinput. Such injected currents could produce severe feedbackinstabilities in addition to permitting the expression of any noisesthat may be included with those injected currents.

Each such opamp 13 and 38 provides, at its output, a signal proportionalto the signal provided at its input, but it provides that signalpotential with a much lower output impedance. Hence, components may thenbe connected to the output of the opamps, such as resistors 53 and 56,which inject interference currents into the output of these amplifiersthat are directly related to the interference potential between therespective source signal grounds 3 and 28 and the output signal ground79. The expression of these currents is suppressed, to a large degree,by the ratio of output impedance of the amplifier(s) 13 and 38 and theimpedance of resistors 53 and 56 respectively. In the preferredembodiment configured for balanced signal sources as shown in FIG.1FIGS. 1A and 1B, such interference due to said opamps is additionallycancelled by the differential action of the circuit comprising opamp 61,and resistors 53, 56, 59 and 57. In this way rejection of the commonmode interference is limited primarily by the precision of the resistors53 and 56 viz-a-viz resistors 57 and 59. This result makes it possibleto utilize smaller values for these resistors and hence makes widerbandwidth and lower noise levels possible.

Additional rejection of such opamp characteristics is offered by theincorporation of opamps 16 and 41, which are especially beneficial whenthe signal source is single ended and comprises only signal 1. In thiscase, the circuit comprised of opamp 16, resistor 14 and resistor 15 maybe altered in design by connecting resistor 39 to the positive input ofopamp 38 instead of the connection shown, and by matching the ratio ofresistors 36 and 35 with the ratio of resistors 40 and 39 so connected.Such an arrangement will produce the same common mode injected errors inopamp 16 as are produced by opamp 13, and these errors will then besubtracted by the differential action of the circuit of opamp 61 incombination with resistors 53 and 55. This improvement could also beapplied to the balanced input source case by applying the aforementionedmodifications to the input circuit for the complementary source.

Each signal presented at the output of opamps 13 and 38 is thenappropriately summed along with inverted signals produced by opamps 16and 41. These signals and their individual complements hence producetwice the voltage level possible between their outputs than the singleopamp 13 or 38 could provide. In addition, this complementary output ispresented symmetrically with reference to the source ground potential asis related by the respective cable shield 4 or 29, thus permitting thecancellation of the common mode interference included in that referencepotential as compared to the destination reference potential. Thisarrangement makes it possible to design the following differential gainstage with opamp 61 and resistors 53-57 and 59 with ½ the gain thatwould otherwise be necessary, and this fact also reduces the sensitivityof the resultant output to that common mode interference. When allcomponent sensitivities are taken into account, a worst case improvementof 4 db in said common mode rejection ratio can be expected withoutincreasing the precision of the resistors required by said differentialgain stage.

In any event, the signal from opamps 13, 38, 41 and 16 are summed by thefollowing difference amplifier stage as follows: The signal from opamp13 is applied to opamp 61 by way of resistor 53 to provide for theexpression of that potential minus the inverted version of thatpotential expressed with reference to the input signal referencepotential as presented by the shield conductor of cable 4 by way ofresistor 55. Likewise, resistor 54 provides for the expression of itsrespective signal potential minus the inverted version of that potentialexpressed about the input signal reference potential ground 28 aspresented by the shield conductor of cable 29 by way of resistor 54.Furthermore, resistor 57 provides for the addition of the outputreference ground 79 as presented by the shield conductor of cable 75 tothe signal output of opamp 61. In this way the output potential of opamp61 may present a potential at its output that not only calculates thedifferences between the input potentials and their ground referencereturns, but also adds in the output reference potential so that thesignal then tracks the reference potential used by the destinationreceiving the output of opamp 61. These relationships expressedmathematically are as follows:

Where

Vout=Output of opamp 61

Vog=Output ground reference potential as presented by the shield ofcable 75

Vin1+=Input from source 1 as presented by cable 4

Vin1g=Input ground reference potential from ground 3 as presented bycable 4

G1=gain of opamp 13=(R10+R11)/R10

VG1+=Output of opamp 13=G1xVin1++Vin1g

VG1−=Output of opamp 16=−G1xVin1++Vin1g

Vin2+=Input from source 26 as presented by cable 29

Vin2g=Input ground reference potential from ground 28 as presented bycable 29

G2=gain of opamp 38=(R35+R36)/R35

VG2+=Output of opamp 38=G2xVin2++Vin2g

VG2−=Output of opamp 41=−G2xVin2++Vin2g

G3=gain of differential amplifier stage with opamp 61=R60 59/R55=R6059/R56=R57/R53=R57/R54

Vog=output ground potential as presented by the shield of cable 75

Vout=output of opamp 61=VG1+−VG1—VG2++VG2−+Vog.

Then the preferred embodiment of the present invention provides for thefollowing relationship:

Signal Output=Vout−Vog=G3x(2xG1xVin1+)−G3x(2xG2xVin2+)=2xG3x((G1xVin+)−(G2xVin2+))

if G1=G2, which is normally the case:

Signal Output=2xG3xG1x(Vin1+−Vin2+)

In order to properly track the output reference potential, while alsodriving the signal with respect to that reference potential, it is alsonecessary to supply opamp 61 with power whose reference return potentialwill accurately assume the same value as the output reference ground 79as presented by cable 75. This is done in this embodiment by providingopamp 61 with its own separate and independent source of power whosereference ground potential can freely assume any value. Transistors 67and 69 provide for such power by implementing very high impedancecurrent sources in the same manner as described previously in connectionwith opamps 13 and 41.

The output of opamp 61 is also filtered, according to the presentinvention to prevent the interaction of high frequencies picked up inthe output cable from being detected by the non-linearities of the opampat those high frequencies. Also, the inductor 72 and resistor 73 serveto provide a finite, but higher impedance to the opamp at higherfrequencies than the capacitor 74 and the capacitance of the cable 75would present at high frequencies, and which would otherwise renderopamp 61 unstable in the servo action of its gain controlling feedback.Also, inductor 72 provides for very low impedance at lower frequenciesso that the output cable 75 can be driven with extremely low impedancewhich can be very effective in shunting any currents that may beinjected by electric fields along the cable, or by the electronic deviceto which the cable may be connected. In addition, the low outputimpedance afforded by inductor 72 makes it possible for opamp 61 to moreaccurately drive any cable capacitance that may be presented by cable75. This is possible because a higher output impedance, as would benecessary without inductor 72, could result in a low pass RC filter (theR being resistor 73 and the C being the sum of capacitor 74 and theeffective capacitance of cable 75) that would have a significant effecton the fidelity of the signal being conveyed. In addition, such lowdrive impedance also shunts the effects of the dielectric of the cable75. This dielectric may not be ideal as it may be subject tohysteresis-like effects such as dielectric absorption. Low outputimpedance will effectively reject such characteristics.

In addition, capacitors 58 and 60 may also be added to minimize thetendency of some opamps to amplify higher frequencies in a manner thatis not consistent with feed back resistor values, and which couldcompromise signal fidelity. Capacitor 60 shunts higher frequencies thatmay be delivered by resistors 55 and 56 while capacitor 58 increases thefeedback applied to opamp 61 while it shunts higher frequencies that maybe delivered by resistors 53 and 54 to the output. To minimize theeffects of these capacitors in the frequency bands of interest,capacitors 58 and 60 should have values that are inversely proportionalto resistors 57 and 59 respectively.

One problem with a signal conditioning apparatus intended to interfacebetween two electronic subsystems is the range of different types ofinputs to which the device will be connected. Destination devices canvary from having a ground connection that is ultimately connected to thesystem ground potential to fully isolated differential connections wherethere is equal, but finite, and sometimes large impedances between thedestination signal ground connection and the system ground. Preferably,the present invention is utilized in a system in which the destinationsignal ground connection is connected to the system power returnpotential. Since this is not always the case with all destinationdevices, however, additional means to guarantee such a connection may beincluded in the present invention.

There is often the occasion to provide signal conditioning for more thanone signal at a time. In such instances arrays of signal conditioningdevices may be required, such that each signal may be processed bycompletely separate signal conditioning apparatus. In such instances thepower supplies are independent in their ability to relate to therespective signal ground potentials.

Since the present invention utilizes only six matched resistors tocondition a balanced source input, it requires only four matchedresistors to condition a single ended source input. Further, because ofthe addition of an inverted circuit placed judiciously with the inputcircuit, an almost two fold increase in the effective interferencerejection of the device may be accomplished for the same resistormatching. As a result, system cost for a given specification is reducedsubstantially.

As it is often desirable to adjust the gain of the subject signalconditioning embodiment, this gain may be adjusted without affecting theabsolute level of unrejected interference. Hence, the present inventionis unique in that common mode forms of interference are rejected with arejection ratio that is actually proportional to the gain so that theinterference residuals of each signal's common mode remains constant inspite of the increased gain. Only common mode interferences betweendifferentially applied signals would be proportional to this gain, butthese interferences are typically very small in relation to the normalcommon mode interferences.

FIG. 2 depicts FIGS. 2A and 2B depict a front end of a typical audioamplifier. A first channel of the audio amplifier includes a first stage100 consisting of opamp 101, resistors 102-106 and capacitor 107 forminga non-inverting opamp circuit. A second stage includes a highpass activefilter circuit 108 and a low pass active filter circuit 116. Switch 124is provided for selectively switching the output of highpass filter 108,lowpass filter 116 or the output of first stage 100 directly, to theinput of opamp circuit 125 consisting of opamp 126 and resistors127-129.

The second channel of the audio amplifier is similar to the firstchannel and includes a first stage 164 consisting of opamp 164,resistors 137-141 and capacitors 142 and 143 also forming anon-inverting opamp circuit. The second stage includes highpass activefilter circuit 165 and lowpass active filter circuit 166. Switch 158selectively switches the output of highpass filter 165, lowpass filter166 or the output of first stage 164 directly, to the input of invertingopamp circuit 167 consisting of opamp 162 and resistors 159-161. Each ofthese circuits is well known in the art and each circuit, therefore,will not be described in detail below. These circuits will collectivelybe referred to hereinafter as the “front end” or preamp of theamplifying system. The output of opamp circuits 125 and 167 can be inputto additional electronic circuitry 133 for power amplification, noisereduction, equalization or providing a noise gate function, etc., or anycombination thereof. The outputs of processing circuitry 133 can then beinput to speakers 134.

According to this embodiment of the present invention, signal source 131and 136 can consist of any desired source of audio signals, such as atuner, tape player, compact disc player, etc. When operating in stereo,signal source 131 can represent a left channel and signal source 136 canrepresent a right channel of an audio signal, for example. Whenoperating in mono, signal sources 131 and 136 can represent the samesignal source. By providing a non-inverted signal on line 132 via opampcircuit 125 and its complement on line 163 via inverting opamp circuit167, it is possible to achieve twice the output voltage by bridging theoutputs of circuitry 133.

First stage opamp circuits 100 and 164 form buffer amplifiers providingat their outputs signals proportional to the signals provided at theirinputs, but with a much lower output impedance.

Highpass filter circuits. 108 and 165 block the low frequency signalsand DC while passing the high frequency signals. These high frequencysignals can be used for driving a high frequency speaker such as a“tweeter”, for example. The output of highpass filter circuit 108 isprovided to terminal A of switch 124 and the output of highpass filtercircuit 165 is provided to terminal A of switch 158.

On the other hand, lowpass filter circuits 116 and 166 block the highfrequency signals while passing the low frequency signals. These lowfrequency signals can be used for driving a low frequency speaker suchas a “woofer”, for example. The output of lowpass filter circuit 116 isprovided to terminal C of switch 124 and the output of lowpass filtercircuit 166 is provided to terminal C of switch 158.

Switches 124 and 158 are manual switches used for directing signals fromone of terminals A-C to the respective inputs of opamp circuits 125 and167. Opamp circuit 125 provides a non-inverted signal on output line132. Inverting opamp circuit 167 provides an inverted signal on outputline 163. The signal on output lines 132 and 163 can then be input toadditional electronic circuitry 133 for power amplification, noisereduction, etc. The signals output from circuitry 133 can then input toright and left speakers 134, for example, when operating in stereo. Whenoperating in mono, the outputs of circuitry 133 can be bridged and inputto a single speaker 134.

The “front end” provides most of the voltage amplification in such anaudio amplifier system and electronic circuitry 133 (e.g., a poweramplifier) provides most of the current amplification.

Some of the problems typically encountered by the audio amplifiercircuitry as described above is that the conductors (e.g., coax cables)tend to introduce interference into the audio signal thus degradingaudio quality. As shown in FIG. 3FIGS. 3A and 3B, the present inventionprovides signal conditioning circuitry, similar to that described abovewith respect to FIG. 1FIGS. 1A and 1B, that eliminates noise introducedby the coax cables, without compromising signal quality. Such benefitsare possible using a very minimum amount of additional circuitry.

FIG. 3 shows FIGS. 3A and 3B show the typical audio amplifier depictedin FIG. 2FIGS. 2A and 2Bincluding signal conditioning circuitry similarto that described above with respect to FIG. 1FIGS. 1A and 1B. Circuit200, consisting of resistors 230-232, transistors 233 and 236, zenerdiode 234 and capacitor 235 and circuit 201 consisting of resistors240-242, transistors 243 and 244, zener diode 245 and capacitor 246provide high impedance constant current sources for delivering the exactcurrent necessary to operate the operational amplifiers in thecircuitry. Constant current source circuits 200 and 201 are illustrativeof circuits that can perform this function. Other well known circuitrymay also be used for delivering constant current. As shown, the groundlines for each of the elements for each channel are tied together andnot to a common ground.

Input amplifier circuits 100 and 164, highpass filters 108 and 165 andlowpass filters 116 and 166 perform the same functions as describedabove with respect to FIG. 2FIGS. 2A and 2B. The present inventionincludes inverting opamp circuit 202 consisting of resistors 203 and 204and opamp 205 and inverting opamp circuit 250 consisting of resistors251 and 252 and opamp 253. The gain of inverting opamp circuits 202 and250 can be varied by varying the resistance ratios of resistors 203 and204 resistors 251 and 252, respectively.

A basic difference in the circuitry between FIG. 2FIGS. 2A/2B and FIG.3FIGS. 3A/3B is that the output signals from switches 124 and 158 andthe output of opamp circuits 202 and 250 are summed by differentialamplifiers 125 and 167, respectively, as follows: The signal from switch124 is applied to opamp 126 by way of the voltage divider consisting ofresistors 127 and 128 to provide for the expression of that potentialminus the inverted version of that potential expressed with reference tothe input signal reference potential as presented by the shieldconductor of cable 130 by way of opamp 205 and resistor 206. Theinverted version of the signal from switch 158 is expressed withreference to the input signal reference potential as presented by theshield conductor of cable 135 by way of opamp 253 and resistor 159 toprovide for the expression of that potential minus the signal fromswitch 158 applied to opamp 162 by way of resistor 160. As describedabove with respect to FIG. 2FIGS. 2A and 2B, the output of opamps 126and 162 may then be further processed by additional electronic circuitry134 (e.g., a power amplifier) and output by one or more speakers 134.

The additional electronic circuitry 133 can be provided as a separateand distinct unit from preamp circuitry 600 depicted in FIG. 3FIGS. 3Aand 3B. In the alternative, the additional electronic circuitry 133 andthe preamp or “front end” circuitry 600 can be provided as a unit in asingle case, for example. Using the circuitry as described in FIG.3FIGS. 3A and 3B, the present invention is capable of eliminating noiseintroduced caused by the conductors and providing a clean output signal.

In addition to conditioning audio signals in an audio amplifier asdescribed above, the signal conditioning circuitry of the presentinvention can also be provided in other arrangements. For thedescription of the following embodiments of the present invention, thesignal conditioning circuitry depicted in FIG. 1FIGS. 1A and 1B(or the“front end” depicted in broken line box 600 in FIG. 3FIGS. 3A and 3B) isshown in block diagram form as block 403.

As shown in FIG. 4, the fight and left outputs of signal source 400 canbe fed through coax cables 401 and 402, respectively, to the input ofpreamp 403. As noted above, preamp 403 can consist of the signalconditioning circuitry depicted in FIG. 1FIGS. 1A and 1B, or can consistof the “front end” circuitry 400 as depicted in FIG. 3FIGS. 3A and 3B.Preamp 403 provides voltage amplification and eliminates noise in thesignals being processed, without compromising signal quality. The outputof preamp 403 can then be provided to the input of power amplifier 410.Power amplifier 410 provides current amplification of the signals onlines 411 and 412 for driving right and left speakers 420 via lines 413and 414 when operating in stereo, for example. When operating in mono,lines 413 and 414 can be bridged to drive a single speaker.

According to this embodiment of the present invention, preamp 403 can beprovided as a separate unit from power amplifier 410, or can be providedas an integral unit with power amp 410. If preamp 403 is provided as aseparate unit, instead of providing individual power supplies for poweramp 403 and power amp 410, power can be supplied to preamp 403 frompower amp 410 via cable 430.

FIG. 5 is similar to FIG. 4, but includes an additional electroniccomponent 500. Component 500 can consist of an equalizer, crossover,noise reduction or effects processor circuitry, etc. or any combinationthereof. The present invention can be used in any state of the artamplification system to eliminate noise without altering the effect ofany other type of processing and without compromising sound quality inany way.

It is preferable that the majority of the voltage amplification of theamplifier system of the present invention take place in preamp 403 andthat any circuitry that reduces gain be provided after preamp 403. Themajority of the current amplification of the amplifier system of thepresent invention occurs in power amp 410. However, it may also bepreferable that speakers 420 include their own current amplificationcircuits, particularly if speakers 420 consist of bass speakers. Inaddition, the present invention makes it possible to provide suchcurrent amplification without the necessity of isolated power supplieswhich are costly and lower in performance. In order to achieve the bestresults, it is preferable that if intermediate processing units areprovided between the voltage amplification circuitry and the currentamplification circuitry, that the power supplies for driving each ofthese units be isolated power supplies.

The signal conditioning circuitry of the present invention can also beused for conditioning other types of signals besides audio signals. Forexample, as shown in FIG. 6, the video signals from camera 600 can befed through coaxial cable 610 to the input of signal conditioningcircuit 620. Signal conditioning circuit 620, consisting of the signalconditioning circuitry depicted in FIG. 1FIGS. 1A and 1B, suppresses oreliminates noise or interference introduced by the coaxial cables andprovides a relatively clean output signal. The output signal can then befed via coax 630 to video display 635 for immediate display and/or toremote video recorder 640 for recording. Signal conditioning circuit 620can be provided as a unit separate from camera 600, display 635 andrecorder 640, or can be incorporated into an input or output stage ofdisplay 635 and/or recorder 640.

The foregoing has set forth exemplary and preferred embodiments of thepresent invention. It will be understood, however, that variousalternatives will occur to those of ordinary skill in the art withoutdeparture from the spirit and scope of the present invention.

What is claimed is:
 1. An amplifier circuit comprising: a first stagefor amplifying at least one applied signal and for generating at leastone intermediate signal proportional to a potential difference betweenthe at least one applied signal and at least one return reference signalcorresponding to the at least one applied signal; a second stage,operatively coupled to the first stage, for frequency filtering theintermediate signal, the second stage generating at least one filteredintermediate signal and an inverted filtered intermediate signal whichis proportional to a potential difference between the filteredintermediate signal and the at least one return reference signalcorresponding to the at least one applied signal; and a third stage forgenerating an output signal which is a sum of the filtered intermediatesignal generated in the second stage minus a potential of the invertedfiltered intermediate signal generated in the second stage.
 2. Anamplifier circuit as recited in claim 1, wherein the first stagecomprises a non-inverting amplifier.
 3. An amplifier circuit as recitedin claim 1, wherein the second stage comprises a highpass filter.
 4. Anamplifier circuit as recited in claim 1, wherein the second stagecomprises a lowpass filter.
 5. An amplifier circuit as recited in claim1, wherein the second stage comprises an inverting opamp circuit forgenerating the inverted filtered intermediate signal.
 6. An amplifiercircuit as recited in claim 1, wherein the second stage comprises alowpass filter for providing a low frequency output and a highpassfilter for providing a high frequency output and means for selectivelyswitching therebetween.
 7. An amplifier circuit as recited in claim 1,further comprising a constant current source for driving the first andsecond stage with a constant current.
 8. An amplifier circuit as recitedin claim 1, further comprising power amplification circuitry forproviding current amplification of the output signal from the thirdstage.
 9. An amplifier circuit as recited in claim 1, wherein the atleast one applied signal is input from a source by a coaxial cable. 10.An amplifier circuit as recited in claim 1, the first stage comprisingan operational amplifier circuit.
 11. An amplifier circuit as recited inclaim 10, wherein the first stage comprises a non-inverting amplifier.12. An amplifier circuit as recited in claim 1, the third stagecomprising an operational amplifier circuit.
 13. An amplifier circuit asrecited in claim 1, wherein the second stage comprises a cross-overcircuit.
 14. A system for amplifying and conditioning signals, thesystem comprising: a first circuit comprising: a) an input for receivingat least one input signal; b) a first stage for generating at least oneintermediate signal which is proportional to a potential differencebetween the at least one input signal and at least one return referencesignal corresponding to the at least one input signal, said first stagefurther generating at least one inverted signal corresponding to the atleast one input signal; and c) a second stage, operatively coupled tothe first stage, for generating an output signal which is a sum of theat least one intermediate signal generated in the first stage plus anoutput return reference minus a potential of the inverted signal; and asecond circuit comprising an amplifier for current amplifying the outputsignal from the second stage of the first circuit.
 15. A system foramplifying and conditioning signals as recited in claim 14, wherein thefirst circuit and the second circuit are provided in separate anddistinct housings.
 16. A system for amplifying and conditioning signalsas recited in claim 15, wherein the second circuit includes a powersupply and supplies power to the first circuit.
 17. A system foramplifying and conditioning signals as recited in claim 14, the firststage further comprising frequency filter circuitry for frequencyfiltering the at least one input signal.
 18. A system for amplifying andconditioning signals as recited in claim 17, wherein the frequencyfilter circuitry comprises a low pass filter.
 19. A system foramplifying and conditioning signals as recited in claim 17, wherein thefrequency filter circuitry comprises a high pass filter.
 20. A systemfor amplifying and conditioning signals as recited in claim 17, whereinthe frequency filter circuitry comprises a low pass filter and a highpass filter and means for selectively switching therebetween.
 21. Asystem for amplifying and conditioning signals as recited in claim 14,wherein the amplifier comprises a power amplifier.
 22. A system foramplifying and conditioning signals as recited in claim 14, wherein thefirst circuit comprises a preamplifier.
 23. A system for amplifying andconditioning signals as recited in claim 14, wherein the amplifiercircuit comprises an amplifier for amplifying video signals.
 24. Asystem for amplifying and conditioning signals as recited in claim 14,wherein the first circuit and second circuit are provided in the samehousing.
 25. A system for amplifying and conditioning signals as recitedin claim 14, further comprising circuitry for equalizing, noise reducingand effects processing the signal output from the second stage and forproviding the output signal to the second circuit.
 26. A circuit foramplifying and conditioning audio signals in an audio system, thecircuit comprising: a) a first stage for generating at least oneintermediate audio signal which is proportional to a potentialdifference between at least one applied audio signal and at least onereturn reference signal corresponding to the at least one audio signal,said first stage further generating at least one inverted applied audiosignal corresponding to the at least one applied audio signal; b) asecond stage, operatively coupled to the first stage, for generating anoutput audio signal which is a sum of the at least one intermediateaudio signal generated in the first stage plus an output returnreference signal minus a potential of the inverted applied audio signal;and c) a power amplifier for current amplifying the output audio signalgenerated in the second stage.
 27. A circuit for conditioning videosignals in a video system, the circuit comprising: a) a first stage forgenerating at least one intermediate video signal which is proportionalto a potential difference between at least one applied video signal andat least one return reference signal corresponding to the at least oneapplied video signal; said first stage further generating at least oneinverted applied video signal corresponding to the at least one appliedvideo signal; and b) a second stage, operatively coupled to the firststage, for generating an output video signal which is a sum of said atleast one intermediate video signal generated in the first stage plus anoutput return reference signal minus a potential of the inverted appliedvideo signal.
 28. A circuit for conditioning signals in a system,comprising: a first stage having a buffer amplifier for receiving atleast one input signal and generating therefrom at least oneintermediate signal which is proportional to a potential differencebetween the at least one input signal and at least one return referencesignal corresponding to the at least one input signal; a power supplycircuit coupled to said first stage for providing a constant currentwith a high impedance to increase electrical isolation between the firststage and the at least one return reference signal; and a second stage,operatively coupled to the first stage, for generating an output signalwhich is a sum of the at least one intermediate signal generated in thefirst stage minus a potential of the at least one return referencesignal plus an output return reference signal.
 29. The circuit accordingto claim 28, further comprising power supply circuitry coupled to saidsecond stage for providing a constant current with a high impedance toincrease electrical isolation between the second stage and the outputreturn reference signal.
 30. The circuit according to claim 29, whereina power return reference potential of said power supply circuit coupledto said first stage is independently determined by a potential of saidreturn reference signal corresponding to said input signal.
 31. Thecircuit according to claim 30, wherein a power return potential of saidpower supply circuitry coupled to said second stage is independentlydetermined by a potential of said output return reference signal. 32.Apparatus for conditioning signals, comprising: a) a first circuit parthaving a first buffer amplifier for receiving a first applied signal andgenerating therefrom a first intermediate signal proportional to apotential difference between said first applied signal and a firstreturn reference signal corresponding to said first applied signal, andhaving a first power supply circuit for providing a constant current tosaid first buffer amplifier with a high impedance, said first powersupply circuit having a first power return reference potentialindependently determined by a potential of said first return referencesignal; b) a second circuit part having a second buffer amplifier forreceiving a second applied signal and generating therefrom a secondintermediate signal proportional to a potential difference between saidsecond applied signal and a second return reference signal correspondingto said second applied signal, and having a second power supply circuitfor providing a constant current to said second buffer amplifier with ahigh impedance, said second power supply circuit having a second powerreturn reference potential independently determined by a potential ofsaid second return reference signal; c) a third circuit part,operatively coupled to said first circuit part, for generating a firstoutput signal which is a sum of said first intermediate signal plus anoutput return reference signal minus a potential of said first returnreference signal; d) a fourth circuit part, operatively coupled to saidfirst circuit part, for generating a second output signal which is a sumof a potential of said second return reference potential plus saidoutput return reference signal minus said second intermediate signal;and e) an output circuit for bridging said first and second outputsignals.
 33. The apparatus according to claim 32, wherein said first andsecond circuit parts further comprise first and second inverters,respectively, for providing respective first and second inverted signalscorresponding to said first and second applied signals, respectively,said first and second inverted signals being applied to the third andfourth circuit parts, respectively, to substantially double voltages inthe output signals thereof.
 34. The apparatus according to claim 32,further comprising a first filter circuit coupled between said firstbuffer amplifier and said third circuit part, for filtering said firstintermediate signal, and a second filter circuit coupled between saidsecond buffer amplifier and said fourth circuit part, for filtering saidsecond intermediate signal.
 35. The apparatus according to claim 34wherein said first and second filter circuits include respective firstand second operational amplifiers driven by respective first and secondconstant current sources each having a high impedance.
 36. A circuit forconditioning signals in a system having a power supply and a powersupply reference potential, said circuit comprising: a first stagehaving an amplifier circuit for receiving, from an input source, aninput signal and an input source reference potential different from thepower supply reference potential, said first stage generating anintermediate signal which represents the sum of the input sourcereference potential and a proportion of the input signal; a first powersupply circuit coupled to said first stage, said first power supplycircuit isolating the power supply from said first stage by drawing aconstant current from the power supply with respect to changes in theinput source reference potential; and a second stage coupled to saidfirst stage, and responsive to said intermediate signal, for generatingan output signal which is proportional to the input signal.
 37. Acircuit for conditioning signals in a system as defined by claim 36,further comprising a second power supply circuit coupled to said secondstage having an output and an output device coupled to the output ofsaid second stage and having an output device reference potentialdifferent from the input source reference potential, said power supplycircuit isolating the power supply from said second stage by drawing aconstant current from the power supply with respect to changes in theoutput device reference potential.
 38. A circuit for conditioningsignals in a system, comprising: a first stage having an amplifiercircuit for receiving, from an input source, an input signal and aninput source reference potential and generating an intermediate signalwhich represents the sum of the input source reference potential and aproportion of the input signal; a first power supply circuit coupled tosaid first stage, said first power supply circuit comprising voltageregulation means for drawing a constant current from the power supplywith respect to changes in the input source reference potential toisolate the power supply from said first stage; and a second stagecoupled to said first stage, and responsive to said intermediate signal,for generating an output signal which is proportional to the inputsignal.
 39. A circuit for conditioning signals in a system as defined byclaim 38, further comprising a second power supply circuit coupled tosaid second stage having an output and an output device coupled to theoutput of said second stage and having an output device referencepotential different from the input source reference potential, saidsecond power supply circuit isolating the power supply from said secondstage by drawing a constant current from the power supply with respectto changes in the output device reference potential.
 40. A signalconditioning circuit in a system having at least one input source, apower supply and an output device, said signal conditioning circuitbeing interposed between the at least one input source and the outputdevice and being coupled to the power supply, the at least one inputsource having a first reference potential, the output device and thepower supply having a second reference potential different from thefirst reference potential, said signal conditioning circuit comprising:a first stage coupled to a second stage; a power supply circuit coupledto the power supply and to said first stage for isolating the powersupply from said first stage by drawing a constant current from thepower supply with respect to changes in the first reference potential;said first stage having an amplifier circuit for receiving an inputsignal and the first reference potential from the at least one inputsource and generating an intermediate signal that represents the sum ofthe first reference potential and a proportion of the input signal; andsaid second stage responsive to said intermediate signal for generatingan output signal which is proportional to the input signal.
 41. A signalconditioning circuit in a system having at least one input source, apower supply and an output device, said signal conditioning circuitbeing interposed between the at least one input source and the outputdevice and being coupled to the power supply, the at least one inputsource having a first reference potential, the output device having asecond reference potential different from the first reference potentialand the power supply having a third reference potential different fromthe first and second reference potential, said signal conditioningcircuit comprising: a first stage coupled to a second stage; a firstpower supply circuit coupled to the power supply and to said first stagefor isolating the power supply from said first stage by drawing aconstant current from the power supply with respect to changes in thefirst reference potential; a second power supply circuit coupled to thepower supply and to said second stage for isolating the power supplyfrom said second stage by drawing a constant current from the powersupply with respect to changes in the second reference potential; saidfirst stage having an amplifier circuit for receiving an input signaland the first reference potential from the at least one input source andgenerating an intermediate signal that represents the sum of the firstreference potential and a proportion of the input signal; and saidsecond stage responsive to said intermediate signal for generating anoutput signal which is proportional to the input signal.
 42. The signalconditioning circuit of claim 41 wherein said first stage generates asecond intermediate signal that represents the difference of the firstreference potential and a proportion of the input signal, said secondstage responsive to said second intermediate signal for generating saidoutput signal from said intermediate signal and said second intermediatesignal.
 43. A signal conditioning circuit in a system having at leastone input source, a power supply and an output device, said signalconditioning circuit being interposed between the at least one inputsource and the output device and being coupled to the power supply, theat least one input source and the power supply having a first referencepotential and the output device having a second reference potentialdifferent from the first reference potential, said signal conditioningcircuit comprising: a first stage coupled to a second stage; a powersupply circuit coupled to the power supply and to said second stage forisolating the power supply from said second stage by drawing a constantcurrent from the power supply with respect to changes in the secondreference potential; said first stage having an amplifier circuit forreceiving an input signal and the first reference potential from the atleast one input source and generating an intermediate signal thatrepresents the sum of the first reference potential and a proportion ofthe input signal; and said second stage responsive to said intermediatesignal for generating an output signal which is proportional to theinput signal.
 44. A signal conditioning circuit in a system having atleast one input source, a power supply and an output device, said signalconditioning circuit being interposed between the at least one inputsource and the output device and being coupled to the power supply, theat least one input source and the output device having a first referencepotential and the power supply having a second reference potentialdifferent from the first reference potential, said signal conditioningcircuit comprising: an amplifier stage having an amplifier circuit forreceiving an input signal and the first reference potential from the atleast one input source and generating an intermediate signal thatrepresents the sum of the first reference potential and a proportion ofthe input signal; and a power supply circuit coupled to the power supplyand to said amplifier stage for isolating the power supply from saidamplifier stage by drawing a constant current from the power supply withrespect to changes in the first reference potential.